A standard hammer or percussion device has a hammer forming a cylinder in which is provided a piston that is longitudinally reciprocal and that defines in the cylinder a rear compartment and a front compartment. This piston moves longitudinally forward to strike the rear end of a tool, for instance a drill bit, and backward to return to a starting position spaced longitudinally behind the tool. The force for forward motion comes from the pressure differential between the front and rear compartment. The force for the return stroke comes in part from this differential and in part from the bit rebounding from the tool.
When such an arrangement is used in a hammer drill where the tool is rotated as well as longitudinally reciprocated, it is standard to provide a sleeve that axially surrounds the tool at the front end of the housing and that is axially reciprocal so that it can hammer back against the tool to assist in withdrawing it when, for instance, a deep hole is being drilled. Such a system is seen in French Patent 2,330,507.
A principal disadvantage of the known such system is that during each stroke the sleeve strikes twice against the housing of the tool itself so this housing absorbs part of the energy of the hammer piston. Clearly this can damage the housing.
In addition the systems known for facilitating withdrawal of the tool require an independent source of fluid under pressure. This makes the apparatus more complicated and expensive, and even increases the amount of energy it uses.